Implement a Scheme REPL based on the Ribbit VM

[mossprescott]

This involved a lot of side projects, including:

• Implement an enhanced CPU with a larger, flat memory space with RAM and ROM both containing both instructions and data. In the process, the hardware simulators were made significantly more general with little apparent performance impact.
• Implement a separate UI harness simulating a character-mapped display to save some RAM (2K for 80x25 characters vs. 16K for 512x256 pixels) and simplify text interaction.
• Improve the compiler in [alt/reg.py] to generate much more compact code, and to provide access to assembly-level addresses and invocations.

[mossprescott]

Current status:

The interpreter largely works, and performance is good enough to call it interactive, provided you used the fast --simulator compiled option, and don't use tracing.

Garbage-collection isn't implemented yet. This will become a priority when the interpreter runs correctly for long enough to fill the heap.

A simple expression like (+ 1 2) can be entered and evaluated. The compiled code looks correct until it tries to call + and then it falls over, so something is not quite right yet.

[mossprescott]

A few bugs have been sorted out, and the getchar/putchar opcodes are now more responsive and support simple line editing.

That's sufficient to make this "usable".

In the REPL, a couple of simple expressions will consume the available memory, so we'll need a garbage collector to make this any more real.

[mossprescott]

Calling this good enough to merge. Planning to add pointer tagging and garbage collection on another branch.